As magnetic storage devices, a memory composed of magnetoresistive elements as storage elements, is known for example.
As an example of the magnetoresistive element, a structure called Tunneling Magnetroregistance (hereinafter referred to as TMR) in which a tunnel insulating film is put between two magnetic bodies, will be described. FIG. 1 is a sectional view showing an example of a TMR element reported by Roy Scheuerlein, et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, 2000 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS (p. 128). This TMR element includes an antiferromagnetic layer 201, a pinned layer 202, a tunnel insulating layer 203, and a ferromagnetic free layer 204, which are laminated. The antiferromagnetic layer 201 is composed of FeMn (10 nm). The ferromagnetic pinned layer 202 is composed of CoFe (2.4 nm). The tunnel insulating layer 203 is composed of Al2O3. The ferromagnetic free layer 204 is composed of NiFe (5 nm). Conductive wiring lines are respectively connected to the antiferromagnetic layer 201 and the free layer 204 such that voltage can be applied thereto. A magnetization direction of the pinned layer 202 is fixed to a certain direction by the antiferromagnetic layer 201. The free layer 204 is formed to easily be magnetized in a certain direction, and the magnetization direction can be changed by externally applying a magnetic field. Among horizontal directions of a film of the free layer 204, a direction into which magnetization is easy is referred to as an easy axis while a direction which is perpendicular to the easy axis and into which magnetization is hard is referred to as a hard axis. An electric current flows through the tunnel insulating film 203 when applying voltage between the free layer 204 and the pinned layer 202, and a resistance value changes depending on the relationship between the magnetization directions of the free layer 204 and the pinned layer 202. That is to say, resistance is low when the magnetization directions are the same while resistance is high when the magnetization directions are opposite directions.
Next, a nonvolatile memory (magnetic storage device) that uses a TMR element as a storage element will be described. FIG. 2 is a perspective view showing an example of a nonvolatile memory reported by M. Durlam, et al., “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, 2000 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS (p. 130). This nonvolatile memory 210 is provided with a pair of wiring lines intersecting above and below each of TMR elements 205 arranged in the form of an array. Upper wiring lines 206 are connected to free layers of the TMR elements 205. Antiferromagnetic layers of the TMR elements 205 are connected to drains of transistors 208 formed in a lower layer through third wiring lines 207. A current is flowed through two wiring lines B (any of B1 to B4) and D (any of D1 to D4) to generate a synthetic magnetic field in the vicinity of the intersection, and a magnetization direction of a free layer is set depending on a direction of the current. Consequently, a resistance value of the TMR element 205 can be changed. Data read can be performed as follows. First, the transistor 208 connected to the TMR element 205 for read is turned to the on state by a wiring lines W. Next, voltage is applied to the TMR element 205 from the wiring line B. Consequently, a current flows through the TMR element 205. Read is performed by evaluating a resistance value of the TMR element 205 with the flowing current.
As a shape of a plane of the aforementioned free layer, a shape that is long in one direction such as an elliptic shape and a rectangular shape is generally used. In such a shape that is long in one direction, an axis into which magnetization is easy, i.e. an easy axis, is formed in the long direction due to shape anisotropy of a magnetic body. A storage element can generate two magnetization states depending on which the magnetization direction of the magnetic body is directed to in the easy axis. A magnetic field that reverses a direction of magnetization is sensitive to shape anisotropy, i.e. a shape of a free layer. In a nonvolatile memory that uses a large number of magnetic storage elements for memory cells however, forming free layers of the same shapes is more difficult as memory cells are miniaturized. As a result, variation is caused in the shapes of the individual free layers and the variation may cause variation in switching fields. In particular, as change in a magnetization direction of a part along a long side is prevented and a magnetic field required for magnetization switching becomes larger, variation is likely to be caused when deformation is present in a pattern of this part. As mentioned above, the disclosed magnetic storage device has a problem that a switching field is large and prevention of variation in switching fields tends to be difficult.
Japanese Laid-Open Patent Application JP-P2001-267522A (corresponding U.S. Pat. No. 6,396,735 (B2)) discloses a magnetic memory element and a magnetic memory. In the magnetic memory element, at least a first magnetic layer, a nonmagnetic layer, and a second magnetic layer are laminated. The magnetic memory element is provided with a third magnetic layer through at least one conductive layer on a side different from the side where the nonmagnetic layer is laminated of the first or second magnetic layer.
Japanese Laid-open Patent Application JP-P2005-85951A discloses a magnetic storage element and a magnetic memory. The magnetic storage element at least includes a storage layer for retaining information based on magnetization states of a magnetic body, and an auxiliary magnetic layer in which a magnetization state changes due to external a magnetic field. The auxiliary magnetic layer includes a plurality of magnetic layers divided by nonmagnetic layers. Magnetic interaction in which magnetizations run antiparallel is present between adjacent magnetic layers of the auxiliary magnetic layer. The total magnetization amount of the even-numbered magnetic layers of the auxiliary magnetic layer and the total magnetization amount of the odd-numbered magnetic layers of the auxiliary magnetic layer are approximately equal.
Japanese Laid-Open Patent Application JP-P2006-352062A discloses a magnetoresistive device and a magnetic memory using the magnetoresistive device. The magnetoresistive element includes a free magnetization layer capable of magnetization switching, and a fixed magnetization layer in which magnetization is fixed. The free magnetization layer includes a plurality of ferromagnetic layers and includes a first antiparallel-coupling synthetic ferrimagnetic structure formed such that two adjacent ferromagnetic layers of the plurality of ferromagnetic layers are antiparallel coupled through a nonmagnetic layer, and a first switching induction layer showing ferromagnetism. The first switching induction layer is formed such that the first switching induction layer is ferromagnetically coupled with the first antiparallel-coupling synthetic ferrimagnetic structure and such that a switching field is smaller than a magnetic field in which the antiparallel coupling of the first antiparallel-coupling synthetic ferrimagnetic structure begins to come off.